In some telephone carrier systems of the particular type mentioned above, the transmission line between carrier terminals is so long that one or more signal-amplifying repeaters are needed to compensate for line attenuation of the carrier signals. Such repeaters are connected to the transmission line at spaced apart locations between the carrier terminal equipment at one end of the line and the carrier terminal equipment at the other end of the line.
Known repeaters for such plural channel, frequency division multiplexed telephone carrier systems are usually self-adjusting within limits for cable or transmission line length between the repeater and the remote signal source which may be, for example, carrier terminal equipment or another repeater. Such repeaters, however, are not automatically self-adjusting for different cable gauges or other factors affecting the cable slope independently of cable or transmission line length. For a composite of frequency division multiplexed carrier signals (i.e., carrier signals of different pre-selected frequencies) transmitted in a common direction, the cable slope is considered to be the db difference between the levels of two different ones of the carrier signals (usually but not necessarily the highest and lowest carrier frequencies) after the composite has been transmitted over the line extending between two terminals or points.
In carrier system repeaters of the type generally described in U.S. Pat. No. 3,475,561 an automatic gain control circuit operates on a composite of FDM (frequency division multiplexed) carrier signals that are transmitted in a common pre-selected direction to derive an AGC (automatic gain control) signal. This AGC signal varies with the d.c. average of the composite carrier signal and is used to provide a slope adjustment for the carrier signals that are transmitted in both directions. In a subscriber or station carrier system, for example, the AGC circuit customarily operates on the carrier composite that is transmitted from the central office terminals, and the resulting AGC signal is used to adjust the levels of both the central office-transmit carrier signals and the subscriber-transmit carrier signals.
Because it is derived from a composite of carrier signals the AGC signal mentioned above does not provide a measure of the cable or carrier signal slope in the sense that it will not be indicative of variations in the slope due to use of different cable gauges or other factors that cause the cable slope to vary independently of transmission line length. Instead, the AGC signal is merely useful as a measure of the transmission line length over which the carrier signal composite has been transmitted, and such length measurement, furthermore, is only accurate at one particular cable gauge.
As such, these prior repeaters are designed for just one particular gauge and will properly adjust within limits for cable length and slope only at the gauge for which they are designed. If they are used with cable having a different gauge, slope errors will develop. When this happens, the potential for crosstalk arises.
If such a prior repeater is used in a subscriber carrier system with a cable gauge other than the one for which it is designed, the crosstalk-producing slope error will occur in the carrier signals that are transmitted to the central office terminal equipment from the subscriber terminal equipment. It is understood that the occurrence of crosstalk is between two or more carrier systems having their separate transmission lines or wire pairs in a common sheath.
Because the AGC circuit of the prior repeaters operates on the composite of carrier signals being transmitted in one direction, a further crosstalk-producing slope error will occur if the majority of the carrier channels for which the system is designed are not in operation for one reason or another. In a typical eight channel carrier system, for example, an unacceptable crosstalk-producing slope error will probably occur if four or more channels are not in operation.
Therefore, at least four, if not more, of the eight channels in an eight channel carrier system must be kept in operation all of the time in order to make the system function properly even in situations where fewer than four channels are needed to serve available subscribers. If a telephone operating company has just two subscribers in need of carrier service, but wishes to provide for later expansion from two to eight channels, it nevertheless must initially buy enough central office channel terminal equipment to operate at least four channels, if not more, just to make the system function properly.
If the slope error abnormally increases the level of any carrier signal arriving at the central office from the subscriber terminal equipment, a central office relay sensitivity problem is created as well as the crosstalk problem mentioned above. Under such circumstances the excessively strong carrier signal will be received in an adjacent channel of the carrier terminal equipment as well as its own designated channel, thus causing operation of the central office relay for the adjacent channel to falsely signal that the subscriber assigned to the adjacent channel has come off hook.
The arrival of excessively strong carrier signals at the central office terminal equipment also creates a channel receive filter problem. This problem and the previously mentioned relay sensitivity problem can be dealt with by making the central office channel filters more selective. Such a solution is not satisfactory because it increases the manufacturing cost of the channel filters.
If the carrier signals become too strong because of slope errors or other reasons, a harmonic distortion problem also occurs with the repeater's amplifiers. To overcome the distortion problem it is necessary to use more expensive amplifiers which have a sufficiently larger dynamic range for handling the abnormally strong carrier signals without causing distortion.
As will become apparent from the following summary and detailed description, the present invention has none of the foregoing drawbacks and offers additional advantages not present in known prior repeaters.